1. Field of the Invention
This invention relates to metal catalysts for the reforming of hydrocarbon fuels, and more specifically, this invention relates to nano-structured noble metal catalysts based on hexametallate architecture for the reforming of hydrocarbon fuels.
2. Background of the Invention
Hydrogen reforming involves gasifying a carbon-containing fuel to produce a gas mixture containing varying amounts of carbon monoxide and hydrogen. More specifically, hydrocarbon reforming is a process used to develop synthesis gas, which in turn is used to synthesize hydrogen, methanol and ammonia.
Reforming is manifested in a myriad of ways. Examples include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen. When starting out with higher carbon feedstocks, those higher hydrocarbons are first cracked to olefins and methane which in turn react further with steam to yield hydrogen and oxides of carbon.
The reforming process is usually carried out over catalysts which comprise one or more metal moieties, most commonly nickel and/or platinum. These metals may be combined with rhenium, tin, or iridium dispersed on an acidic support. Catalysts are combined with various support materials, including but not limited to aluminia, calcium aluminate, magnesia alumina, and combinations thereof. Alumina is a preferred support due to its versatility.
Catalytic activity is closely related to both the amounts and strength of acidic active sites distributed over the catalyst surface. Very weak acidic active sites may not be very catalytically active, and strongly acidic active sites can lead to excessive hydrocarbon cracking and carbon deposition on the surface.
Excessive carbon deposition can also occur due to aggregation of catalytic metal atoms to form larger crystals (Ostwald ripening). Sintering and sublimation of catalytic metal atoms under hydrocarbon cracking conditions can also be a problem. Ostwald ripening can be alleviated by relative isolation of individual catalytic metal atoms. Sintering and sublimation can be solved by strong attractions holding the catalytic metal atoms to the support material's crystal lattice.
A. The “Coking” Problem
Carbon deposition on the surface of a reforming catalyst is a continual and serious problem in the reforming of hydrocarbon fuels. Carbon deposition leads to a decrease in catalyst activity via the blocking of active sites. Forms of carbon known as whisker carbon can cause physical depletion of catalytic sites which in turn gives discontinuation of the catalytic process due to “reactor plugging” or poisoning of the catalytically active sites. Carbon deposition onto the surface of a conventional reforming catalyst occurs primarily through the dehydrogenation of strongly adsorbed hydrocarbons into polynuclear aromatic compounds known as “coke” and also through the pyrolytic decomposition of hydrocarbons.
The structure of the catalyst surface is defined by the dispersion of the catalytic metal and the coordination number of the active sites which contain the catalytic metal. Both of these factors, dispersion and coordination number, have been shown to affect the adsorption of hydrocarbons onto the surface of a catalyst. Reactions which lead to carbon deposits on catalysts are more likely on surfaces where the hydrocarbons have been strongly adsorbed. This strong adsorption occurs on electron rich surfaces such as those found in noble metals. The opposite effect, weak adsorption (i.e. surfaces more Lewis base-like which do not seek electrons) is found on electron deficient surfaces.
B. The Sulfur Problem
Sulfur compounds present in hydrocarbon feeds quickly deactivate or poison catalysts by preferentially adsorbing onto catalytically active sites which results in the sites' occlusion. At low ratios of hydrogen (H2) to hydrogen sulfide (H2S), hydrogen sulfide can also react with the active catalytic metals to form an inactive metal sulfide.
Sulfur compounds adsorb very strongly onto electron rich surfaces such as the surfaces of noble metals and weakly onto electron deficient surfaces.
The inventor has previously developed metal substituted hexaaluminate catalysts wherein the catalyst is directly doped with transition metals such as nickel, rhodium, and vanadium. That work is disclosed in U.S. patent application Ser. No. 11/390,216, filed on Mar. 28, 2006, by the instant assignee and incorporated herein by reference. By dispersing the metal crystallites such as nickel over the surface of an electron deficient support, carbon deposition is minimized and a strong metal-support interaction is created which prevents the metal crystallites from aggregating, sintering, and subliming.
U.S. Pat. No. 7,166,268 awarded to Fukunaga on Jan. 23, 2007 discloses a hydrocarbon reforming process and a hydrocarbon reforming catalyst. The catalyst comprises an alumina (Al2O3) carrier containing cerium oxide (CeO). The surface contains two required components “a” and “b”, and third one “c” optional. Component “a” comprises at least one platinum group element selected from ruthenium Ru, platinum, rhodium, palladium, and iridium. Component “b” is cobalt Co and/or nickel Ni. Component “c” is an alkaline earth metal.
U.S. Pat. No. 7,150,866 awarded to Wieland, et al. on Dec. 19, 2006 discloses a process for the autothermal, catalytic steam reforming of hydrocarbons and a catalyst for said process. The catalyst has a multilayer structure and comprises a lower catalyst layer located directly on a support body and an upper catalyst layer located on the lower catalyst layer. Each catalyst layer comprises at least one platinum group metal on an oxidating support material.
U.S. Pat. No. 7,067,453 awarded to Ming, et al. on Jun. 27, 2006 discloses a catalyst consisting of an oxide or mixed oxide support and bimetallic catalytically active compounds selected from platinum, and ruthenium, and prepared in an appropriate ratio.
U.S. Pat. No. 6,958,310 awarded to Wang, et al. on Oct. 25, 2005 discloses two catalysts. The first catalyst comprises a first porous structure upon a second porous spinel structure and a steam reforming catalyst disposed upon the second porous surface.
U.S. Pat. No. 6,905,998 awarded to Naka et al. on Jun. 14, 2005 discloses a catalyst performance recovery method for a reforming catalyst apparatus. The method entails heating the catalyst to a temperature ranging from 500° C. to 800° C.
U.S. Pat. No. 6,884,340 awarded to Bodgan on Apr. 26, 2005 discloses a process for the catalytic reforming of a naphtha feedstock and two different catalysts. The first catalyst has a refractory inorganic oxide support with small portions of a platinum-group metal component, a tin (Sn) and/or germanium (Ge) component, an indium (In) component, and a lanthanide metal component. The second catalyst is similar to the first catalyst, but has only germanium as the second metal component and the lanthanide component is cerium or lanthanum (La).
U.S. Pat. No. 6,808,652 awarded to Park, et al. on Oct. 26, 2004 discloses a modified alumina (Al2O3)-supported nickel reforming catalyst which has improved coke resistance, and high-temperature catalytic stability and activity.
U.S. Pat. Nos. 6,294,492 and 6,291,381 awarded to Lin on Sep. 25 and Sep. 18, 2001, respectively, disclose an activation method for a catalytic reforming catalyst. A catalyst with carbon or coke depositions is treated by heating and contact with hydrogen gas (H2) and organic chlorine-containing compounds. The contact reactivates the catalyst.
U.S. Pat. No. 6,171,992 awarded to Autenrieth et al. on Jan. 9, 2001 discloses a treatment process for a methanol reforming catalyst which pre-ages the catalyst so there will not be any noticeable decrease of the catalyst's efficiency due to a reduction of the catalyst's activity as a result of a decrease in the catalyst's volume. The method includes heating the catalyst to a temperature ranging from 240° C. to 350° C.
None of the aforementioned patents discloses a nano-structured noble metal catalyst wherein the noble metal atoms are isolated via their replacement of support crystal lattice metal atoms.
In addition, none of the aforementioned patents discloses a nano-structured catalyst wherein mirror cations of the lattice structure are chosen to reduce surface Lewis acidity. Therefore, none of the aforementioned patents claims aid in the oxidation of carbon deposits.
Also, none of the aforementioned patents discloses a nano-structured catalyst wherein the catalytic noble metal atoms are in electron deficient environments which aid in the prevention of both carbon depositions and the formation of inert metal sulfides.
Further, none of the aforementioned patents discloses a reforming catalyst which does not need pre-treatment before use or reactivation after use, i.e., a maintenance-free catalyst.
Also, none of the aforementioned patents disclose a method for hydrocarbon fuel reforming which employs a noble metal-doped hexametallate catalyst which is resistant to carbon deposition and the formation of inert metal sulfides.
Finally, none of the aforementioned patents disclose a method for preparing nano-structured hexametallate catalysts having a spinel block, and noble metal atoms dispersed throughout the interior of the lattice in electron deficient environments.
A need exists in the art for a nano-structured noble metal catalyst wherein the noble metal atoms are isolated via their replacement of support crystal lattice metal atoms.
A need also exists in the art for a nano-structured catalyst wherein mirror cations of the lattice structure are chosen to reduce surface Lewis acidity and aid in the oxidation of carbon deposits.
In addition, a need exists in the art for a nano-structured catalyst wherein the catalytic noble metal atoms are in electron deficient environments which aid in the prevention of carbon depositions and also of the formation of inert metal sulfides, in particular, catalyst with superior resistance to carbon deposits and metal sulfide formation.
Further, a need exists in the art for a reforming catalyst which does not need pre-treatment before use or reactivation after use, i.e., a maintenance-free catalyst.
In addition, a need exists in the art for a method for hydrocarbon fuel reforming which employs a noble metal-doped hexametallate catalyst which is resistant to carbon deposition and the formation of inert metal sulfides.
Finally, a need exists in the art for a method for preparing nano-structured hexametallate catalysts having a spinel block, and noble metal atoms dispersed throughout in electron deficient environments.